Oscillating RPM question

My truck (99 tacoma 3.4L 4x4 manual transmission) starts and runs. Idles around 2000 until its warm and then drops to 1000. No problem there. Different story when driving. Runs at normal rpms on the highway but when the clutch is depressed (say when exiting) the rpms either seem to stay stuck up around 2200 or drop down to where I would expect (around 1000) but then oscillate between 1000 and 1800 (almost like the engine in revving). Only the clutch is being pushed. No gas. When the truck is put back into gear after the stop, the engine returns to normal rpms and drives normally. Any ideas on possible causes? If i haven't explained the situation clearly please let me know.

Thanks